Return-Path: Received: (qmail 31048 invoked from network); 20 Jan 2003 19:22:18 -0000 Received: from warrior.services.quay.plus.net (212.159.14.227) by mailstore with SMTP; 20 Jan 2003 19:22:18 -0000 Received: (qmail 1752 invoked from network); 20 Jan 2003 19:21:31 -0000 Received: from post.thorcom.com (193.82.116.70) by warrior.services.quay.plus.net with SMTP; 20 Jan 2003 19:21:31 -0000 X-SQ: A X-Priority: 3 X-MSMail-Priority: Normal Received: from majordom by post.thorcom.com with local (Exim 4.12) id 18ahTS-0000N6-00 for rsgb_lf_group-outgoing@blacksheep.org; Mon, 20 Jan 2003 19:20:46 +0000 Received: from [147.197.200.9] (helo=hestia.herts.ac.uk) by post.thorcom.com with esmtp (Exim 4.12) id 18ahTR-0000Mx-00 for rsgb_lf_group@blacksheep.org; Mon, 20 Jan 2003 19:20:45 +0000 X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2800.1106 Received: from gemini ([147.197.200.44] helo=gemini.herts.ac.uk) by hestia.herts.ac.uk with esmtp (Exim 3.22 #1) id 18ahSt-00029y-00 for rsgb_lf_group@blacksheep.org; Mon, 20 Jan 2003 19:20:11 +0000 Received: from [147.197.232.252] (helo=rsch-15.herts.ac.uk) by gemini.herts.ac.uk with esmtp (Exim 3.33 #1) id 18ahSi-0006tU-00 for rsgb_lf_group@blacksheep.org; Mon, 20 Jan 2003 19:20:00 +0000 Message-ID: <5.1.0.14.0.20030120171137.00b158e0@gemini.herts.ac.uk> X-Sender: mj9ar@gemini.herts.ac.uk X-Mailer: QUALCOMM Windows Eudora Version 5.1 Date: Mon, 20 Jan 2003 19:14:42 +0000 To: rsgb_lf_group@blacksheep.org From: "James Moritz" In-reply-to: <004401c2c0a2$25622860$9801a8c0@gigacom.com.br> References: <23.2aadfb3f.2b5c5522@aol.com> MIME-Version: 1.0 X-MailScanner: No Virus detected Subject: Re: LF: Loops x Q Content-Type: text/plain; charset=iso-8859-1; format=flowed Content-Transfer-Encoding: 8bit X-Spam-Status: No, hits=-0.4 required=6.0tests=DEAR_SOMEBODY,IN_REP_TO,REFERENCES,SPAM_PHRASE_02_03version=2.43 X-SA-Exim-Scanned: Yes Sender: Precedence: bulk Reply-To: rsgb_lf_group@blacksheep.org X-Listname: rsgb_lf_group X-SA-Exim-Rcpt-To: rsgb_lf_group-outgoing@blacksheep.org X-SA-Exim-Scanned: No; SAEximRunCond expanded to false Dear Marcus, LF Group, It seems to me this is quite a large loop, and will generate a large output: I think across an untuned loop V = 2.1e-8.f.N.A.E where N = turns, f is frequency in Hz, A is area in m^2, E is f/s in V/m For your loop, and a f/s of 1uV/m, this gives 0.21uV @ 400k, 0.85uV @ 1600k., With a Q of 200, the voltage at resonance will be increased to 42uV @ 400k, 170uV @ 1600k The loop noise depends on the RX bandwidth, but in 10kHz BW it will be roughly 6uV @ 400k, 12uV @ 1600k. So for 1uV/m, SNR would be 17dB @ 400k, 23dB @ 1600k. The JFET noise will be negligible. I think most of the time the atmospheric noise level in 10k BW at MF will be well above 1uV/m, so it should be more than sensitive enough. Reducing the Q to 100 due to resistors or varicap will make the output voltage decrease by 6dB and loop noise decrease by 3dB - still probably OK. Reducing the RX bandwidth will improve the SNR, of course. The tuning is a problem - 4:1 tuning range needs 16:1 capacitance variation, including the stray capacitance of the loop. If L is about 446uH from the table, this means 350p - 22p. The stray capacitance of the loop may be greater than 22p, so you may not get the full tuning range. Using a varicap diode, it is unlikely the minimum- to maximum capacitance ratio will be high enough. it may be necessary to reduce the number of turns and use a larger tuning capacitor. The other problem is that local stations will give very large signals - eg. 10mV/m from an 1MHz MF broadcast station will result in about 1V across the loop at resonance, which will probably lead to severe intermods from both the preamp and the varicap. You could connect the preamp across only 1 turn instead of all 9 - this would still give several uV out for 1uV/m f/s, more than enough for a decent receiver. In fact, since the loop output is so large, you could probably eliminate the preamp by using a single turn coupling loop to a 50ohm RX input. The air-variable tuning capacitor would also be a good idea for this reason, unless you live a very long way from any broadcast stations! Hope this is helpful, Cheers, Jim Moritz 73 de M0BMU At 13:35 20/01/2003 -0300, you wrote: >Hi ... > >After read an article last year (Wireless World) about loop antennas X DCF39 >calibration, I constructed a octogonal RX loop, #16 awg wire, to cover from >400KHz to 1.6MHz. > >ThereŽs 9 turns, 5mm spacing, 1.9 meter diameter. About 55 meter of wire. > >This is the first version. Later IŽll build another one to 136KHz. > >Acording with this article, I should expect a Q=200 ... 220 from such loop. > >I considered the NTE618 to tune it and a push-pull pre-amp (J310) with gates >conected to the loop ends. No resistors there. The article consider that for >such loop at this frequency will have something like .20uV of thermal noise. >The resistors will add some extra noise there so I removed them. > >But the Q=210 ... and the NTE618 Q is 200. > >Is there any gain If I use a dual-gang air variable driven by a step motor + >reduction in place of the NTE618?